The invention relates to the turn-off of a gate-controlled switch, and particularly to the turn-off of a gate-controlled switch at a reduced rate, i.e. to what is known as a soft turn-off.
Power inversion is commonly implemented using a switch configuration in which each output phase is provided with a pair of switches connected in series between the positive and negative voltages of a direct voltage circuit, whereby an output phase branch connected to a point between the switches of each switch pair may be provided with inverted power from the direct voltage circuit.
In frequency converters and other inverters there may arise a situation where due to a malfunction the upper and lower semiconductor switches of an inverting switch pair are simultaneously conductive and therefore an extremely high current from the capacitor battery of the intermediate circuit passes through the semiconductor switches, the current being restricted by nothing but the characteristics of the semiconductor switches. This is referred to as breakthrough.
In IGB (Insulated Gate Bipolar) transistors, for example, breakthrough current may be restricted to a range that equals 5 to 10 times the nominal current, and the component sustains such current for about ten microseconds without damage. Consequently, it is necessary to succeed in switching off the current within this time. In practice the reason for breakthrough is that one of the semiconductor switches in the inverting switch pair does not obey the control and the current must be switched off with the correctly functioning switch in the branch.
However, breakthrough current must not be cut off too abruptly, because the couplings always contain stray inductance which in connection with a high rate of change in the current may generate a voltage peak which is so high that it destroys the semiconductor switches. The voltage peak û thus generated has the following formula:û=uo−di/dt,
where uo represents the intermediate circuit voltage and di/dt the rate of change of the current. This shows that the only way to restrict the amplitude of the voltage peak is to reduce the rate at which the current drops, i.e. to carry out what is known as a soft turn-off.
A characteristic of switches provided with an insulated gate is that the gate electrode contains capacitance against both the emitter (source) and the collector (drain) electrode. Due to these capacitances a predetermined minimum electrical charge has to be supplied to the gate to ensure that the semiconductor switch changes to a fully conductive state and, correspondingly, a charge of a similar magnitude has to be removed from the gate to open the switch. Moreover, switching-on and switching-off are phenomena that are affected by what is known as the Miller capacitance between the gate and the collector. The Miller capacitance is shown in the form of what is known as the Miller plateau in the gate voltage in situations where the gate charge is increased or decreased.
FIG. 1 shows the mutual dependency between a gate charge Qgate and a gate voltage Vge in a typical IGB-type transistor. In FIG. 1 the gate voltage Vge is 10V at the Miller plateau. In a turn-on situation, with the gate voltage Vge at the Miller plateau, collector current is substantially constant although the gate charge Qgate increases. Correspondingly, in a turn-off situation the gate voltage Vge remains on the substantially constant value of the Miller plateau for a while although the gate charge Qgate decreases. In other words, the decrease in the collector current in practice stops for the time the gate voltage Vge is at the Miller plateau.
A simple way to carry out a soft turn-off is to allow the gate charge Qgate of the semiconductor switch to be discharged through a suitable turn-off resistor either to the emitter potential or to a negative turn-off potential. This is a non-linear process, because the crossing of the Miller plateau takes fairly long, and during that time breakthrough current hardly changes at all. The resistance of the turn-off resistor employed is typically a few hundred ohms.
The turn-off method described above usually requires that a separate soft turn-off controller and a soft turn-off signal are used to connect the turn-off resistor to the circuit for a suitable length of time, such as 100 μs, before a normal turn-off, known as a hard turn-off, is carried out. Alternatively, the turn-off resistor may be allowed to be permanently connected to the circuit, in which case it consumes a lot of energy from the gate driver circuit whenever the gate voltage of the semiconductor switch is positive, which in turn requires a higher power supply capacity in the gate driver than in a configuration without a turn-off resistor. Both a separate soft turn-off controller and a gate driver of a higher capacity cause extra costs.
It is also known in the art to reduce the gate voltage by a separate control circuit in a controlled manner in such a way that the Miller plateau is crossed fast and the breakthrough current decreases almost at a constant rate. However, this requires the use of an expensive analog circuit and a signal to either control a soft turn-off or to convey a completed soft turn-off to the control logic in another potential. Configurations of this type are typically even much more expensive than a soft turn-off involving a turn-off resistor.